Protective devices for electric power transmission systems

ABSTRACT

A device for use in detecting earth faults in a polyphase electric power transmission system by monitoring the difference (e.g., in phase) between a selected parameter and polarising parameter which has a predetermined relation with a voltage which, for a fault to earth at a point at a distance not greater than said predetermined distance along the line from said device, has a value equal to the zero-sequence voltage at that point.

United States 7 Patent 1 191 Perez-Cavero Aug. 5, 1975 I PROTECTIVEDEVICES FOR ELECTRIC 3.192.442 6/1965 Warrington et al 317/36 D POWERTRANSMISSION SYSTEMS 3.519.884 7/l970 Paddison et al. 317/36 D 3,651,3773/1972 [75] Inventor: L nar P r Stafford, 3.732.464 5/1973 Miki et111. 1. 317/36 D England [73] Assignee: The General Electric CompanyPrimary Examinew-James D. Trammell Limited, London England Attorney,Agent, or F irmKirschstein, Kirschstein, 122 Filed: Nov. 27, 1973 OmngeFrank 1211 Appl. No.: 419,289

[57] ABSTRACT l l Foreign Application Priority Data A device for use indetecting earth faults in a poly- Nov. 28. I972 United Kingdom 54834/72phase electric power transmission system by monitoring the difference(e.g., in phase) between a selected [52] US. Cl. 317/27 R; 317/36 Dparameter and polarising parameter which has a pre- [5 l 1 Int. Cl. H02h3/26 determined relation with a voltage which, for a fault to {58] Fieldof Search 317/36 D, 27 R; 324/83 R. earth at a point at a distance notgreater than said pre- 324/83 Q determined distance along the line fromsaid device, has a value equal to the zero-sequence voltage at that [56]References Cited point.

UNITED STATES PATENTS 6 Cl 4 D F 3.024.389 3/1962 Warrington 317/36 Drawmg gums 11 o- I I --C'll Tl v l 4 A Squarer 'Phw! Ll l P (R) to mo.

CTZ VTZ b Squarer Phase l l P Co L (Y) S 'D =1 5 t uarer 111551 1 ll lopl l 1 Comp.

1 y Phase square:

in shifter vpol 1d PATENTED Ans 1975 5 UNEI" v Q0 /10 (Y) I n SquarerSquarcr Phase Shifter Mum-Input Coincidence Comparator Square! Square?(R) .iqu arer (Y) Fig. 2

squarer Shifter fVpol Fig.3

PATENTED NIB 5W5 3, 898.530

sum 3 26 Z v Squaw? Phase Comparalor MP squarer 27 Z H {v squarer PhaseComparator 0!) Squarer (Y) o Q 21 1 3:1 l I vre m 25 B W i l u V Squarer8! Phase comparator Vop(B) Squarer (B) 9 as 12 F A 39 V; VsquarerPROTECTIVE DEVICES FOR ELECTRIC POWER TRANSMISSION SYSTEMS Thisinvention relates to devices for use in detecting the occurrence ofearth faults in a polyphase electric power transmission system.

In operation some such devices monitor the difference, e.g., in phase oramplitude, between one or more selected parameters and a so-calledpolarising parameter.

The present invention resides in the use of a new polarising parameterwhich reduces the occurrence of malfunctions.

According to the invention in a device for use in detecting theoccurrence of earth faults within a predetermined distance of the devicealong a polyphase electric power transmission system comprising meansfor monitoring the difference between at least one selected parameterand a polarising parameter, the polarising parameter has a predeterminedrelation with a voltage which, for a fault to earth at a point at adistance not greater than saidpredetermined distance along the line fromsaid device, has a value equal to the zerosequence voltage at thatpoint.

The point on the line at said predetermined distance along the line ishereinafter called the reach point and the point on the line at whichthe device is positioned is hereinafter called the monitoring point.

The fault point zero-sequence voltage which occurs for a fault at apoint is given by the expression:

1/3 RES RES rn) where:

V is the residual voltage at the monitoring point,

I is the residual current at the monitoring point,

Z is the zero-sequence impedance of the system between said device andsaid fault point.

It will be appreciated that the desired polarising parameter is readilyobtainable from quantities available at the monitoring point.

In some applications of the invention the polarising parameternecessarily has a predetermined relation with a voltage which has avalue equal to the fault point zerosequence voltage for a fault at thereach point, i.e., Z,.,, in the equation above must be equal to thezerosequence impedance of the system between the device and the reachpoint (2 In other applications the polarising parameter may have apredetermined relation with a voltage which has a value equal to thefault point zero-sequence voltage for a fault at any particular point ata distance from the device not greater than the reach point (i.e., ZZ,,,,).

The invention finds application in devices of the kind I wherein themonitoring means comprises a respective phase comparator means for eachphase of the system. each comparator means monitoring the phasedifference between the polarising parameter and a selected parameter ofthe corresponding phase of the system.

The invention also finds application in devices of the kind wherein themonitoring means comprises a multiinput phase comparator means whichmonitors the overall phase difference between the polarising parameterand a selected parameter of each phase of the system. i

The invention also finds application in devices of the kind wherein themonitoring means comprises a respective amplitude comparator means foreach phase of the system, each amplitude comparator means monitoring thedifference between a first derived quantity and a second derivedquantity the first derived quantity being the difference between thepolarising parameter and a selected parameter of the corresponding phaseof the system, and the second derived quantity being the sum of thepolarising parameter and a selected parameter of the corresponding phaseof the system.

Several arrangements in accordance with the invention will now bedescribed by way of example with reference to the accompanying drawingsin which:

FIG. I is a schematic diagram of a device for detecting the occurrenceof earth faults on each phase of a three-phase electric powertransmission system, the de vice exhibiting a reactance typecharacteristic;

FIG. 2 is a schematic diagram of a device for detecting the occurrenceof earth faults in a three-phase electric power transmission system, thedevice exhibiting a mho-reactance type characteristic;

FIG. 3 is a schematic diagram of a modified form of the device shown inFIG. 2; and

FIG. 4 is a schematic diagram of a self-polarised device for detectingthe occurrence of earth faults on each phase of a three-phase electricpower transmission system, the device exhibiting a mho typecharacteristic.

Referring to FIG. 1, the first device to be described includes threevoltage transformers VTl, VT2 and VT3 to whose primaries arerespectively applied the red yellow and blue phase to earth voltages VVy and V of the system at the monitoring point.

The device further includes three current transformers CTl, CT2 and GT3each of which has a first primary winding through which passes arespective one of the three phase currents I l and l at the monitoringpoint, and a second primary winding through which passes the residualcurrent I at the monitoring point. Each of the transformers CTl, CT2 andCT3 has connected across its secondary a replica impedance Z havingavalue equal to that of the positive sequence impedance per phase of thesystem between the monitoring point and the reach point of the device(i.e., the far end from the monitoring point of the section of thesystem over which the device is designed to detect earth faults),referred to the secondary level.

Each of the impedances Z is connected in series with the" secondary ofthe corresponding voltage transformer VTl, VT2 or VT3 betweena commonpoint N and a respective one of three points A, B and C. The turnsratios of the windings of the transformers are chosen so that compoundvoltages Vop(R), Vop(Y) and Vop(B) appear respectively between thepoints A, B, C and point N, which voltages are given by the followingequationsi Vop( vR 2.. (1 K nes) V0110) Y l.1( r+ K nns) P( n 1: (1-H KIRES) where and Z1, is the zero-sequence impedance of the system betweenthe monitoring point and the reach point of the device. referred to thesecondary level.

The voltages Vp(R). Vop( Y) and V0p( B) are respectively applied viathree squarers l, 2 and 3 to first inputs of three phase comparators 4,5 and 6.

The voltages V0p(R) Vop( Y) and V0p(B) are combined via equal resistors7 to produce a voltage Vx which is applied via a phase shifter 8 and asquarer 9 to a second input of each of the comparators 4, 5 and 6. Fromequations l (2) and (3) above it can be seen that Vx V0p(R) V0p( Y)V0p(B) V (I 3K nes) n (5) substituting for K from equation (4) gives VXnss u) nes When the fault occurs at the reach point, the voltage Vx isequal to 3 V0, where V0 is the zero-sequence voltage at the fault point.

The phase shifter 8 is arranged to provide a phase shift of qSr. Thevalue of r will usually be:

where 4),, is the argument of the zero-sequence impedance of the system,which argument varies with the power system conditions between a maximumvalue 4),, max and a minimum value (1),, min. Other values of tar may beused in alternative arrangements.

The output of the phase shifter constitutes a polarising voltage V forthe device, an earth fault on a phase in the section of the systemprotected by the device being indicated when the corresponding phasecomparator detects a phase difference between its first and secondinputs of greater than 90.

It will be appreciated that due to the variation of with power systemconditions, the length of the section of the system over which thedevice will respond to earth faults with a certain arc resistanceincreases or decreases as I00 0 becomes greater or less than (1),, i.e.,the device overreaches or underreaches. However. the extent to whichthis occurs is appreciably less than in known devices wherein otherpolarising parameters such as phase current or zero-sequence current atthe monitoring point are used.

Another advantage of the device shown in FIG. I is that it operatescorrectly even if the system is not earthed behind the relay point, thatis with no zerosequence current at the relay point.

Referring now to FIG. 2, in the second device to be described. voltagesV0p(R V0p( Y), V0p(B) and VpoL are applied via respective squarers 10 to13 to respective inputs ofa multi-input coincidence comparator 14. Thevoltages V0p(R), Vup(Y) and VuptB) are conveniently derived in the samemanner as in FIG. I and the polarising voltage VpoL is convenientlyderived from the combination of voltages Vop via a phase shifter 15, asin FIG. I.

In operation of the device an earth fault on any phase in the section ofthe system protected by the device is indicated when the comparatorproduces an output indicating that the four inputs to the comparatorhave an overall phase difference of less then 180. The use of thevoltage VopL as polarising parameter instead of a voltage in phase withthe zero-sequence current at the relay point has the same advantages asmentioned above with regard to FIG. 1.

One difficulty experienced with the device shown in FIG. 2 is that itmay respond to certain earth faults occurring behind the monitoringpoint. Such maloperation can be prevented by applying to a fifth inputofthe comparator 14 a second polarising voltage VpOL ofthe kind proposedby the invention as shown in FIG. 3. This voltage VpoL' is suitably thevoltage at the input of phase shifter 15 and is applied to thecomparator via a further squarer 16. An earth fault is then indicatedwhen the overall phase difference of all five inputs of the comparator14 is less than I".

Alternatively the voltage VpoL may be any other voltage given by theexpression RES RE-s ro where Z is the zero-sequence impedance of the system between the relay point and a point at a distance from the relaypoint not greater than the reach point of the device.

It will be appreciated that maloperation in respect of earth faultsbehind the monitoring point could also be prevented in this manner in adevice using a voltage in .phase with the zero-sequence current at themonitoring point as the primary polarising parameter. instead of VpoL.

Referring now to FIG. 4, the fourth device to be described comprisesthree voltage transformers VT4, VTS and VT6, three current transformersCT4, CTS and CT6 and three replica impedances Z connected in the mannerdescribed above with reference to FIG. 1 to produce, from the phase toearth voltages V V,- and V,,, the phase currents I I, and I and theresidual current I the three voltages V0p(R). V0p( Y) and Vop(B) definedabove in equations (I), (2) and (3).

The three voltages Vop are respectively applied via three squarers l7,l8 and 19 to first inputs of three phase comparators 20. 21 and 22.

The voltages V V and Vy at the secondaries ofthe voltage transformersVT4, VTS and VT6 are respectively applied via squarers 23, 24 and 25 toinputs of three two-input AND gates 26, 27 and 28.

The voltages V0p(R). V0p(Y) and V0p(B) are combined via equal resistors29 to produce the voltage Vx defined above in equation (6). and thisvoltage is applied via a squarer 30 to the second input ofeach of thegates 26. 27 and 28, the outputs of the gates being respectively appliedto second inputs of the phase comparators 20, 21 and 22.

The phase voltages (V Vy and V,,) with the polarity reversed and thevoltage Vx serve as polarising voltages for the device. an earth faulton a phase in the section of the system protected by the device beingindicated when the corresponding phase comparator detects a phasedifference of more than 90 between its first and second inputs.

The use of the second polarising voltage V): prevents maloperation ofthe device for earth faults behind the relay point. The operation of thedevice is based on the fact that the two polarising voltages and theoperating voltage can never be in the same half plane (in a voltagediagram) for earth faults behind the relay point.

It will be appreciated that the second polarising voltage may be anyvoltage given by the expression (8) above. However, the best resultswill normally be obtained when Z and hence the polarising voltage is aslarge as possible. that is when Z Z as in the arrangement of FIG. 4.

In a modification of the arrangement shown in FIG. 4 the AND gates 26,27 and 28 are in the circuits of the operating voltages V0p(R), V0p( Y)and V0p(B) instead of the phase voltages V V,- and V It will beappreciated that the principle of operation embodied in the arrangementof FlG. 4 can be used with types of protective device other than thatshown incFlG. 4, and can, for example, be used with soundtween a firstderived quantity and a second derived quantity the first derivedquantity being the difference between the polarising parameter and aselected parameter of the corresponding phase of the system, and thesecond derived quantity being the sum of the polarising parameter and aselected parameter of the corresponding phase of the system.

I claim:

1. A device for use in detecting earth faults within a predetermineddistance of the device along a polyphase electric power transmissionsystem comprising: means for generating a polarising voltage which has apredetermined relation with a voltage which, for a fault to earth at apoint at a distance not greater than said predetermined distance alongthe line from said device, has a value equal to the zero-sequencevoltage at that point; means for generating a plurality of furthervoltages, one for each phase of the system, each of which furthervoltages has a predetermined relation with the polarising voltage whenan earth fault occurs; and monitoring means for detecting the occurrenceof said predetermined relations between the polarising voltage and saidfurther voltages.

2. A device according to claim 1 wherein said monitoring means comprisesa plurality of phase comparators, one for each phase of the system, eachcomparator having a first input to which said polarising voltage isapplied and a second input to which the further voltage of thecorresponding phase of the system is applied, each comparator beingarranged to detect the occurrence ofa predetermined phase relationbetween its inputs,

3. A device according to claim 1 wherein said monitoring means comprisesa multi-input phase comparator having a first input to which saidpolarising voltage is applied and a plurality of further inputs to eachof which the further voltage of a different phase of the system isapplied, the comparator being arranged to detect the occurrence of apredetermined overall phase relation between its inputs.

4. A device according to claim 1 wherein said means for generating saidfurther voltages produces a respective compound voltage in respect ofeach phase of the system of the form RES) where Vp represents the phasevoltage at the monitoring point Ip represents the phase current at themonitoring point I representsthe residual current at the monitoringpoint Z represents'the positive sequence impedance per phase between themonitoring point and the reach point Z represents the zero-sequenceimpedance between the monitoring point and the reach point N is thenumber of phases in the system.

5. A device according to claim 4 wherein said means for generating apolarising voltage comprises means for combining said compound voltages.

6. A device according to claim 5 wherein said means for generating apolarising voltage includes a phaseshifter which changes the phase ofthe voltage obtained by combining the compound voltages by an amountdependent on the argument of the zero-sequence impedance of the system.

1. A device for use in detecting earth faults within a predetermineddistance of the device along a polyphase electric power transmissionsystem comprising: means for generating a polarising voltage which has apredetermined relation with a voltage which, for a fault to earth at apoint at a distance not greater than said predetermined distance alongthe line from said device, has a value equal to the zero-sequencevoltage at that point; means for generating a plurality of furthervoltages, one for each phase of the system, each of which furthervoltages has a predetermined relation with the polarising voltage whenan earth fault occurs; and monitoring means for detecting the occurrenceof said predetermined relations between the polarising voltage and saidfurther voltages.
 2. A device according to claim 1 wherein saidmonitoring means comprises a plurality of phase comparators, one foreach phase of the system, each comparator having a first input to whichsaid polarising voltage is applied and a second input to which thefurther voltage of the corresponding phase of the system is applied,each comparator being arranged to detect the occurrence of apredetermined phase relation between its inputs.
 3. A device accordingto claim 1 wherein said monitoring means comprises a multi-input phasecomparator having a first input to which said polarising voltage isapplied and a plurality of further inputs to each of which the furthervoltage of a different phase of the system is applied, the comparatorbeing arranged to detect the occurrence of a predetermined overall phaserelation between its inputs.
 4. A device according to claim 1 whereinsaid means for generating said further voltages produces a respectivecompound voltage in respect of each phase of the system of the form
 5. Adevice according to claim 4 wherein said means for generating apolarising voltage comprises means for combining said compound voltages.6. A device according to claim 5 wherein said means for generating apolarising voltage includes a phase-shifter which changes the phase ofthe voltage obtained by combining the compound voltages by an amountdependent on the argument of the zero-sequence impedance of the system.